DivX Movie too dark

I have downloaded a DivX movie that is way too dark.
Is there a way to “re-engineer” the brightness and contrast of the movie without having to re-encode it back to mpg or avi first. Re-encoding the movie would degrade the quality (and that would be a pity)
I could, of course, adjust the monitor settings, but that would be a fix-up rather than a permanent cure.

This is a problem that I have never had before, so any suggestions would be greatly appreciated.

Regards
John